Diseases such as multiple sclerosis reduce the effectiveness of neural transmission. A common diagnostic technique in these diseases is to measure electrical potentials evoked in response to various forms of stimulation of peripheral sensory nerves: abnormally long neural conduction times being equated with neural disease. Neurones concerned with conducting neural signals over longer distances are typically assisted in their conduction by a sheath of insulative material known as myelin. Diseases such as multiple sclerosis disrupt the myelin sheath and thereby impair the speed of neural conduction. Such changes in nerve conduction latency and other changes in the time-course of the evoked potential can be observed in a variety of other disorders such as optic neuritis, various optic atrophies, toxic amblyopia, papilledema, Parkinson's, tumours, migraine, various ataxias, compression of the visual nerves, spinocerebellar degenerations and Vitamin B.sub.12 deficiency as described in the paper by S. Sokol, entitled "The visually evoked cortical potential in the optic nerve and visual pathway disorders", which was published in Electrophysiological testing in diseases of the retina, ootic nerve, and visual pathway, edited by G. A. Fishman, published by the American Academy of Ophthalmology, of San Francisco, in 1990, Volume 2, Pages 105-141.
The evoked potentials (EPs) conventionally measured reflect the activity of large bundles of neurones contained within a nerve. Thus the EP represents a gross sum of many cells' activity. Such a gross sum would tend to mask the effects of small focal lesions to smaller subsets of neurones within the nerve, such as those found in multiple sclerosis, since responses from neurones with damaged and intact myelin are summed together. Therefore it would be desirable to obtain different EP responses produced by different component parts of a nerve in order to highlight focal neural damage.
In more recent times Magnetic Resonance Imaging has provided a method to obtain images at least of the larger focal lesions found in multiple sclerosis. Studies comparing the relative sensitivities and specificities of MRI with a variety of Evoked Potential (EP) methods, including Visual (VEP), Auditory (AEP) and Somatosensory (SEP) methods, reveal that MRI is superior to the EP methods in diagnosing MS as described in the paper by T. Sand and I.A. Sulg, entitled "Evoked potentials and CSF-immunoglobulins in MS: relationship to disease duration, disability, and functional status", which was published in Acta Neural Scand, Volume 82, Pages 217-21, and the paper by HI.A van Diemen, P. Lanting, J.C. Koetsier, R.L. Strijers, H.K van Walbeek and C.H. Pornan, entitled "Evaluation of the visual system in multiple sclerosis, a comparative study of diagnostic tests", which was published in Clin Neurol Neurosurge, Volume 94, Pages 191-5. Of these various EP methods the VEP comes closest to matching the performance of MRI.sub.1, VEP sensitivity rarely lagging MRI by more than 10% as described in the paper by M. Ravnborg, R. Liguori, P. Christiansen, H. Larsson and P.S. Sorensen, entitled "The diagnostic reliability of magnetically evoked motor potentials in multiple sclerosis", which was published in Neurology. Volume 42, Pages 1296-301. As pointed out above part of the failing of the EP methods is undoubtedly that the measured potentials commonly represent a sum over the whole of whichever particular sensory pathway is stimulated. In the case of the VEP some differential measurement is often attempted by using stimuli consisting of checker-board patterns of different scales, the idea being that finer patterns bias the VEP somewhat towards measurements from the central retina and visual field as described in the paper by M.R. Harter, entitled "Evoked cortical responses to checkerboard patterns; effect of check-size as a function of retinal eccentricity", which was published in Vision Res, Volume 10, Pages 1365-76. Attempts have been made to characterise responses from each hemifield separately and to try to achieve some separation of hemispheric responses by use of widely displaced pairs of electrodes as reported in the paper by L.D. Blumhardt, G. Barrett, A.M. Halliday and A. Kriss, entitled "The effect of experimental `scotomata`on the ipsilateral and contralateral responses to pattern-reversal in one half-field", which was published in Clin. Neurophvsiol., Volume 45, Pages 376-392. Inadequate isolation of these responses, in conjunction with different recording electronics and different recording times for the compared responses contribute to less than satisfactory results. Nevertheless, given the close concordance between VEP and MRI.sub.1, an improved VEP, provides the best promise of performance comparable to MRI that could be done in the average neurologist's surgery as often as desired and at lower cost.